Isomyosin expression patterns during rat heart morphogenesis: an immunohistochemical study

An immunohistochemical study of cardiac alpha and beta myosin heavy chain (MHC) expression during rat heart morphogenesis was performed. In tubular hearts (embryonic days, ED10-11) coexpression of both cardiac alpha and beta MHC was found throughout the heart, except for the left free wall of the atrium, where only cardiac alpha MHC is detected. A transition of coexpression to single expression of either cardiac alpha or beta MHC begins at the same time in both atria and ventricles but requires a longer time for completion in the ventricules; in the atria transition takes place during the period ED 12-13 and in the ventricles during ED12-15. Furthermore, expression of cardiac alpha and beta MHC was detected in the sinus venosus, and cardiac alpha MHC expression was detected in the pulmonary veins. A comparison of the results obtained in chicken embryos revealed that in tubular hearts the expression pattern is similar, whereas in later developmental stages two major differences were observed: 1) transition of coexpression to single expression in rat ventricles appears to take a longer developmental period; 2) the persistence of areas of coexpression in the sinoatrial junction, dorsal mesocardium, atrioventricular junction, and outflow tract, as found in the chicken embryo in later developmental stages, is not found in the rat heart.

The gamma-glutamyltransferase/glutamine synthetase activity ratio. A powerful marker for the acinar origin of hepatocytes

The activity of glutamine synthetase (GS) in hepatocytes isolated by digitonin-collagenase perfusion from the perivenous region was more than 10-times higher than in cells isolated from the periportal region. This distribution was confirmed by immunohistochemical staining for GS of cells separated from either region. In contrast, in periportal hepatocytes, the activity of gamma-glutamyltransferase (GGT) was 3-4 times as high as in perivenous hepatocytes. This acinar distribution was also confirmed histochemically. The striking reciprocal acinar distribution of these two enzymes, now observed by direct biochemical analysis of selectively isolated hepatocytes, confirms the earlier qualitative differences observed by histochemistry and immunohistochemistry. The GGT/GS ratio seems to serve as a powerful marker of the acinar origin of isolated hepatocyte populations. Preliminary data describing glutamine synthetase activity in plasma of some subjects with suspected liver dysfunction suggests this enzyme as a marker for pericentral damage.

Localization of ammonia-metabolizing enzymes in human liver: ontogenesis of heterogeneity

Immunohistochemical analysis of human liver (8 to 94 years) shows a compartmentation of ammonia-metabolizing enzymes across the acinus. The highest concentration of carbamoylphosphate synthetase (ammonia) is found in the parenchymal cells around the terminal portal venules. Glutamine synthetase is found in a small pericentral compartment two to three cells thick. In contrast to observations in rat liver, in human liver a well-recognizable intermediate zone can be distinguished in which neither enzyme can be detected. This intermediate zone is not yet established at the age of 8 years but can be recognized in livers from 25 years onward. Carbamoylphosphate synthetase can already be detected in the liver of human fetuses at 5 weeks of development. The enzyme distribution reveals a random heterogeneity among the hepatocytes, suggesting that not all hepatocytes start to accumulate carbamoylphosphate synthetase at the same time. From 9 weeks of development onward, the enzyme becomes homogeneously distributed throughout the liver parenchyma until at least 2 days after birth. Glutamine synthetase cannot be detected during this period. In addition, the definitive architecture of the acinus is not yet completed at birth. These results therefore support the idea that in human liver, metabolic zonation with respect to NH3 metabolism exists as it does in rat liver. Furthermore, the data show that this functional compartmentation becomes established concomitant with the development of the acinar architecture.(ABSTRACT TRUNCATED AT 250 WORDS)

Creatine kinase isozyme expression in embryonic chicken heart

The distribution pattern of creatine kinase (EC 2.7.3.2) isozymes in developing chicken heart was studied by immunohistochemistry. Creatine kinase M, which is absent from adult heart, is transiently expressed between 4 and 11 days of incubation. During that period, numerous muscular cells in the roof and septum of the atrium, in the interventricular septum and on top of the trabeculae cordis and at the rim of the outflow tract stain strongly with a polyclonal antibody that is specific for the M subunit. In the ventricle and outflow tract, the M-positive cells are found mainly subendocardially and in the right half, at the transition of conducting and working myocytes. Creatine kinase B, which is the predominant adult isozyme, is initially expressed to a high concentration in a small group of disperse myocardial cells in upstream part of the inflow tract. When compared to the expression pattern of cardiac myosin heavy chains, the observed creatine kinase expression pattern suggests that M-positive cells are mainly found in areas that participate in the formation of cardiac conductive tissue, whereas B-positive cells are first found in areas that are involved in the generation of cardiac rhythm.

Normal-sized thyroglobulin messenger ribonucleic acid in Dutch goats with a thyroglobulin synthesis defect is translated into a 35,000 molecular weight N-terminal fragment

The translation product of the thyroglobulin (Tg) mRNA in Dutch goats with a Tg synthesis defect has been characterized. The Tg mRNA has a normal size of 8.4 kilobases. Translation of goitrous polysomal Tg mRNA resulted, after immunoprecipitation with polyclonal rabbit antigoat Tg antibodies, in a single 35,000 mol wt (Mr) Tg fragment. To characterize the Tg antigens produced in vivo, thyroid hormone release by the goiter was suppressed by injecting T4 sc in newborn goitrous goats. Immunohistochemical studies showed the presence of Tg antigens almost solely in the colloidal lumen. Electrophoresis of the reduced thyroid proteins demonstrated the presence of two Tg fragments with Mr of 40,000 and 32,000, respectively; the latter is probably a breakdown product of the 40,000 Mr fragment. The difference in Mr between the in vivo and in vitro translation products (40,000 and 35,000 Mr, respectively) can be explained by the carbohydrate content (10% wt/wt) of the in vivo product, as was shown by periodic acid-Schiff-positive staining. Using monoclonal antibodies against the hormonogenic sites at the first and last parts of the Tg molecule, we demonstrated that only the first part of the Tg molecule is present. Both in vivo and in vitro 10% of the goitrous Tg mRNA molecule is translated, resulting in an N-terminal Tg fragment that easily aggregates to large S-S complexes in the colloidal lumen of goiter by H2O2 oxidation.

Induction of glutamine synthetase and transient co-expression with carbamoylphosphate synthetase in hepatocytes transplanted into fat pads of syngeneic hosts

Isolated rat hepatocytes were transplanted into the interscapular and both anterior lateral fat pads of hepatectomized syngeneic rats. At various time points following transplantation, the fat pads were removed, fixed and embedded in paraffin. Serial sections were stained for glutamine synthetase (GS) and carbamoylphosphate synthetase (CPS) using specific antisera and the PAP technique. The initially low fraction of GS+-heptatocytes remained low up to the fourth day, then increased strikingly up to almost 100% and declined gradually after the 14th day. In contrast, the number of CPS+-cells declined continuously to about 30% after 28 days. If the animals were exposed to CCl4 prior to the isolation of the hepatocytes in order to reduce the number of GS+-cells in the initial cell suspension similar results were obtained and no difference in the probability of the colony formation was noted between this and the normal hepatocyte suspensions indicating that the appearance of the GS+-phenotype was not due to a selective survival of these cells. Analysis of the staining intensity of the transplanted hepatocytes revealed the appearance of two populations of GS+-hepatocytes, one with a strong and one with a weak staining, during the course of formation of larger nodules, while only a single weakly stained population could be discerned with respect to the staining for CPS. These results demonstrate that all hepatocytes or at least their descendents can be induced to express GS by the environmental conditions of the fat pads, and that GS and CPS can be co-expressed with an apparently reciprocal relationship.

Gene expression in derivatives of embryonic foregut during prenatal development of the rat

Proteins characteristic for the adult cellular phenotype, i.e., carbamoylphosphate synthetase (CPS) for liver and small intestine, arginase for liver, glutamate dehydrogenase (GLDH) for pancreas, liver, and small intestine, and amylase for pancreas were studied immunohistochemically in rat embryos and fetuses. At distinct developmental stages, subsets of enzymes appear synchronously in the foregut derivatives, suggesting that gene expression in the different organs is regulated by common factors. In contrast to the long-held opinion that fetal hepatocytes are a homogeneous cell population, it is shown that arginase and CPS are heterogeneously distributed between ED 16 and ED 20. This heterogeneity is related to the vascular architecture of the liver and disappears perinatally as the result of strong stimulation of enzyme synthesis. In addition, an intercellular heterogeneity in CPS content that is not related to the vasculature is observed between ED 14 and ED 20. This "random" heterogeneity reflects temporal differences in the onset of CPS accumulation in individual cells.

Complementary distribution of carbamoylphosphate synthetase (ammonia) and glutamine synthetase in rat liver acinus is regulated at a pretranslational level

We studied the distribution of the mRNAs for carbamoylphosphate synthetase (ammonia) and glutamine synthetase in frozen sections of adult rat liver by in situ hybridization to [35S]-labeled cDNA probes. The density of silver grains resulting from hybridization to the labeled cDNA probe for carbamoylphosphate synthetase is highest around the portal venules, decreases towards the central venule, and is virtually absent from an area two to three cells wide that lines the central venules in which mRNA for glutamine synthetase is predominantly localized. Therefore, both mRNAs show the same complementary distribution within the liver acinus that was found for the proteins they encode, demonstrating that compartmentalization of the expression of these enzymes is controlled at a pretranslational level. In addition, we found that carbamoylphosphate synthetase mRNA is present mainly in the epithelium of the crypts of the proximal part of the small intestine, whereas carbamoylphosphate synthetase protein is present in the epithelium of both crypts and villi.

Noradrenergic innervation of developing rat and spiny mouse liver. Its relation to the development of the liver architecture and enzymic zonation

The development of noradrenergic innervation of rat liver was studied with a polyclonal antiserum against noradrenaline. Nerves are first seen in the larger portal vessels at day 1 after birth and reach their final distribution at 5 days after birth i.e. at the same time as the establishment of the acinar architecture and the heterogeneous distribution of NH3-metabolizing enzymes. The latter distribution of nerves is already seen at birth in the liver of the closely related but precocial spiny mouse. This shows that the onset of extrinsic sympathetic innervation is regulated by the developmental stage of the animal rather than by adaptation to extrauterine life. Chemical sympathectomy at birth with 6-hydroxydopamine did not eliminate the developmental appearance of heterogeneous distributions of NH3-metabolizing enzymes.

Immunohistochemical localization of glutamate dehydrogenase in rat liver: plasticity of distribution during development and with hormone treatment

In adult rat liver, glutamate dehydrogenase is present in high concentrations around the terminal portal (zone 1) and hepatic (zone 3) veins, whereas its concentration is low in the intermediate zone. Although the size and staining intensity of the periportal glutamate dehydrogenase-positive compartment are less than those of the pericentral compartment, it can expand under appropriate endocrine conditions, leading to a homogeneous distribution. At birth, glutamate dehydrogenase is also homogeneously distributed. Glutamate dehydrogenase disappears from the periportal compartment during the first postnatal week and reappears in that compartment after weaning. These observations indicate an independent regulation of glutamate dehydrogenase levels in the periportal and pericentral zone. The size of the periportal glutamate dehydrogenase-containing zone is appreciably smaller than that of carbamoylphosphate synthetase, whereas the pericentral glutamate dehydrogenase-containing zone is appreciably larger than that of glutamine synthetase. The heterogeneous distribution of glutamate dehydrogenase suggests the possibility that, under normal conditions, deamination of glutamate prevails in the periportal compartment and amination of glutamate in the pericentral compartment.

Isomyosin expression patterns in tubular stages of chicken heart development: a 3-D immunohistochemical analysis

The 3-D distribution of atrial and ventricular isomyosins is analysed in tubular chicken hearts (stage 12+ to 17 (H/H)) using antibodies specific for adult chicken atrial and ventricular myosin heavy chains, respectively. At stage 12+ (H/H) all myocytes express the atrial isomyosin; furthermore, all myocytes except those originally situated in the dorsolateral wall of the sinu-atrium coexpress the ventricular isomyosin as well. Moreover, it appears that recently incorporated myocardial cells at both ends of the heart tube start with a coexpression of both isomyosins. From stage 14 (H/H) onwards a regional loss of expression of one of either isomyosins is observed in the atrial and ventricular compartment. In this way the single isomyosin expression types that are characteristic for the adult working myocardium of the atria and ventricles arise. So, the isomyosin expression patterns are, unexpectedly, hardly useful to discriminate the different heart parts of the tubular heart. The ventricle, defined by its adult type of isomyosin expression, is even not detectable before stage 14 (H/H). Interestingly, interconnected coexpression areas, which may be precursor conductive tissues, are still present at stage 17 (H/H) in the outflow tract, the ventricular trabeculae, the atrio-ventricular transitional zone and in the sinu-atrium. The pattern of isomyosin coexpression was found to correlate with a peristaltoid contraction and a slow conduction velocity, whereas single expression areas correlate with a synchronous contraction and a relatively fast conduction velocity. The possible implications of the changing isomyosin pattern for the differentiation of the tubular myocardium, in particular in relation to the development of the conductive tissues, will be discussed.

Development of enzymic zonation in liver parenchyma is related to development of acinar architecture

The appearance of the distribution patterns of the NH3-metabolizing enzymes carbamoylphosphate synthetase, glutamate dehydrogenase, and glutamine synthetase in the developing liver of an altricial species (rat) was compared with that in the developing liver of a closely related, precocial species (spiny mouse). The comparison showed that the development of hepatic acinar architecture, rather than perinatal adaptation, is responsible for the development of periportal and pericentral compartments of gene expression. Conditions that confine the expression of specific enzymes to the pericentral compartment of the acinus originate before conditions that confine the expression of (other) specific enzymes to the periportal compartment. However, whether or not the site of gene expression is restricted to specific compartments within the liver acinus, the rate of expression of the gene involved can also be adaptively regulated. Therefore, different factors appear to control the site and the rate of gene expression within one tissue.

The histone H1(0)/H5 variant and terminal differentiation of cells during development of Xenopus laevis

The maintenance of the differentiated condition is supposed to be associated with the presence of a histone of the H1(0)/H5 subclass. If the H1(0)/H5 variant has an important role in differentiation distinct from that of H1, it should display differential expression in time and position during development. Here we report that this prediction is verified during Xenopus laevis development, in which tadpoles exhibit a very characteristic, developmentally regulated pattern of histone H1(0)/H5 expression that is different for the derivatives of each embryonic germ layer. However, the pattern of appearance of this variant during development does not reflect a simple correlation between its presence and the state of differentiation. Therefore, these results are pertinent to current ideas on differentiation and the involvement of lysine-rich histones in the repression of eukaryotic genes.

Isomyosin expression in developing chicken atria: a marker for the development of conductive tissue?

Isomyosin expression patterns in embryonic chicken atria during the first two weeks of development were analyzed immunohistochemically. In the 3-days embryonic chicken heart (HH19-20), strong coexpression of both isomyosins can be found as band-like zones at the lateral sides of the sinoatrial junction. The zones converge on the bottom of the atrium and continue as a band around the atrioventricular canal. In the 5-days heart (HH27-28) the coexpression area encompasses the entire sinoatrial junction and extends into parts of the sinus venosus and into the dorsocaudal atrial wall. In the 7-days heart (HH 32-33) the relative extension of coexpression areas reaches its maximum. Coexpression is also found in a ring-like band in the ventral (bottom) wall of the atria peripheral to the ring-like band in the atrioventricular junction. The latter band has now become continuous with the coexpression area in the bottom of the interatrial septum. Caudally coexpression extends behind the atrioventricular cushions towards the interventricular septum and cranially coexpression of the atrioventricular junction has become continuous with that of the ring around the outflow tract (cf Sanders et al. 1986). In the second week of incubation a decrease of coexpression is observed. The isomyosin expression pattern described in this study has put forward additional arguments that the conductive tissue originates from areas that continue to express both isomyosins relatively late in development.

Immunohistochemical analysis of the distribution of histone H5 and hemoglobin during chicken development

We used immunohistochemical procedures to investigate embryonic erythropoiesis in serial sections of chicken embryos after 2-13 days of incubation. Antibodies specific for the erythrocyte-specific histone H5, for embryonic hemoglobin, and for adult hemoglobin were used as markers for general, primitive, and definitive erythropoiesis, respectively. Histone H5 was present in erythrocytes at all of the stages studied, i.e., in both the primitive and definitive cells. Cell of the definitive lineage were first detected, at about 5-6 days of incubation, in erythroid foci in the mesenchyme around the vitelline stalk. At 7-9 days of incubation, a massive mesenchymal conglomeration of erythropoietic cells developed, extending from the cervical to the abdominal region and ventrally to the vertebral body, with its largest extensions being around the arteries in the mediastinum. Immunostaining revealed that these erythroid cells belonged to the definitive erythropoietic lineage. These cells had disappeared completely after 12 days of incubation, i.e., before erythropoiesis is visible in the bone marrow. These observations are consistent with the notion that the yolk sac is essential for the formation of the definitive erythroid lineage.

Reciprocal regulation of glutamine synthetase and carbamoylphosphate synthetase levels in rat liver

In glucocorticosteroid-treated diabetic rats, glutamine synthetase enzyme levels in the liver are decreased 3-fold, whereas carbamoylphosphate synthetase enzyme levels are increased 2.3-fold. In addition, immunohistochemistry shows that under these conditions the distribution of carbamoylphosphate synthetase is expanded over the entire liver acinus, whereas that of glutamine synthetase is reduced to very few cells bordering the central (terminal hepatic) veins. Using a newly isolated cDNA complementary to rat liver glutamine synthetase mRNA, we show that this regulation is primarily effected at a pretranslational level. (For data on carbamoylphosphate synthetase mRNA levels, see De Groot et al. (1986) Biochim. Biophys. Acta 866, 61-67). Furthermore, hybridization studies show stimulatory effects of both glucocorticosteroids and thyroid hormone on the glutamine synthetase mRNA level. Attempts to localize glutamine synthetase mRNA within the liver acinus by selective destruction of the pericentral zone failed because of generally low levels of liver mRNAs after CCl4 poisoning. In contrast to the situation after birth, significantly higher glutamine synthetase mRNA/enzyme activity ratios in fetal rat liver point to the presence of additional post-transcriptional control mechanisms before birth. These findings complement similar observations on carbamoylphosphate synthetase gene expression (De Groot et al. (1986) Biochim. Biophys. Acta 866, 61-67).

Acetylcholinesterase in prenatal rat heart: a marker for the early development of the cardiac conductive tissue?

In rat embryos, acetylcholinesterase (AChE, EC 3.1.1.7) activity is present in a continuous sleeve of myocytes that extends from the myocardium that is adjacent to the atrioventricular endocardial cushions via the ventricular trabeculae to the outflow tract. No activity is found in the atrial roof, in the ventricular walls and in the interventricular septum except for its subendocardial surface. AChE-positive cells are first identified in 11-day rat embryos, while the prototypical distribution is best demonstrable in 13-day embryos. Part of the AChE-positive cell system is identifiable as a precursor of the adult conduction system by topographical criteria in 16-day fetuses and by morphological criteria in 20-day fetuses. At birth (2 days later), AChE activity has disappeared from the cardiac myocytes except for a ring of tissue at the atrial side of the atrioventricular junction. These findings suggest that the embryonic heart can be divided into an upstream myocardium that has no AChE activity and a downstream myocardium that is characterized by the presence of AChE. Furthermore they suggest that an acetylcholine-dependent mechanism may be responsible for the retardation of the depolarization wave in the downstream parts of the heart. Finally they show that the adult conduction system is formed by a transdifferentiation of part of a far more extensive embryonic precursor system.

Heterogeneous distribution of glutamine synthetase during rat liver development

Two days before birth, immunohistochemical detection of glutamine synthetase already reveals a heterogeneous distribution pattern related to the vascular architecture of the liver. Only a small number of hepatocytes in the vicinity of the efferent venules show relatively high staining intensity. Before that age, only megakaryocytes show intense staining, while liver parenchyma is only faintly stained. The developmental profile of glutamine synthetase activity shows two periods of increasing enzyme activity: one in the perinatal period and one in the second and third postnatal week. Both periods are correlated with high levels of circulating corticosteroid hormones. Although the relative number of intensely stained hepatocytes increases during the first rise in enzyme activity, the second rise is correlated with a decreasing number of glutamine synthetase-positive hepatocytes which, however, show a considerable increase in staining intensity. Carbamoylphosphate synthetase shows a homogeneous distribution pattern in the perinatal period. Conditions that lead during development to a relatively high level of glutamine synthetase expression in the pericentral compartment apparently originate before the appearance of conditions that lead to a relatively high level of carbamoylphosphate synthetase gene expression in the periportal compartment. Our results indicate that downstream localization of glutamine synthetase in liver acinus is essential from the perinatal period onwards, whereas reciprocal distribution of glutamine synthetase and carbamoylphosphate synthetase gene expression (that is found in adult rat liver) is not.

Immunological evidence for an H1(0) type of histone protein in chicken liver

We prepared monoclonal antibodies against chicken histone H5. These antibodies could be divided into two classes, and we present the results obtained with one representative antibody of each class. One class reacted exclusively with chicken H5, whereas the other additionally cross-reacted with rat H1(0) and with material present in adult but not embryonic chicken liver. The cross-reacting material in adult liver was identified by Western blotting as representing a minor band in histone preparations. The protein was not present in histone extracts from chicken erythrocytes. It is likely that this newly identified protein is a chicken H1(0) histone.

Developmental and hormonal regulation of carbamoyl-phosphate synthase gene expression in rat liver: evidence for control mechanisms at different levels in the perinatal period

Carbamoyl-phosphate synthase gene expression is found to be primarily regulated by conditions that enhance hepatic glucocorticosteroid levels (hormone injections) and cyclic AMP levels (induction of diabetes). After birth, changes in the level of carbamoyl-phosphate synthase protein follow changes in the level of carbamoylphosphate synthase mRNA, suggesting a pretranslational control mechanism. In fetal rats, carbamoyl-phosphate synthase gene expression is regulated by the same factors as in adults. However, both the level to which carbamoyl-phosphate synthase mRNA can accumulate and the extent to which mRNA can be translated appear to be limited, indicating control mechanisms at the pretranslational and translational level. Finally, in the immediate postnatal period, a transient but pronounced decrease in the rate of degradation of carbamoyl-phosphate synthase protein may play a role in the accumulation of the enzyme.

The local expression of adult chicken heart myosins during development. II. Ventricular conducting tissue

The development of the ventricular conducting tissue of the embryonic chicken heart has been studied using a previous finding that morphologically recognizable atrial conducting tissue coexpresses the atrial and the ventricular myosin isoforms. It is found that, by these criteria, at 9 days part of the ventricular conduction system consists of a myocardial ring located around the infundibula of the aorta and truncus pulmonalis. Part of this ring is formed by the retro-aortic root branch. The ring continues via the septal branch into the atrioventricular bundle and its branches, that all express both myosin isoforms. The retro-aortic root branch could be traced back as a part of the myocardial wall of the truncus arteriosus at the 4 days embryonic stage. At the 16th day of development, the septal branch, atrioventricular bundle and left and right bundle branches no longer express the atrial isomyosin, but two bundles originating from the septal branch still express both isomyosins, one being the retro-aortic root branch, the other being only immunologically recognizable and directed to the ventral side of the truncus pulmonalis; this latter we call the pulmonary root branch. Both bundles are remnants of the myocardial ring.

Development of the heterogeneous distribution of carbamoyl-phosphate synthetase (ammonia) in rat-liver parenchyma during postnatal development

Carbamoyl-phosphate synthetase (ammonia) is homogeneously distributed in rat-liver parenchyma at birth, as demonstrated by immunohistochemistry. A heterogeneous distribution can first be demonstrated at 6 days post partum, but can be masked by use of a too sensitive detection system. This heterogeneity is established by a decrease in enzyme content around the hepatic venules and a considerable increase in enzyme content in the remaining parenchyma. The perivenous decrease in enzyme content does not occur in all hepatocytes synchronously. The adult type of heterogeneity is characterized by a perivenous layer, only two to three cells thick, in which carbamoyl-phosphate synthetase can no longer be detected, irrespective of the sensitivity of the assay used. This situation is fully established at the age of two months.

The conducting tissue in the adult chicken atria. A histological and immunohistochemical analysis

A three-dimensional reconstruction from serial sections of adult chicken heart was made to verify whether Purkinje cells, that can be recognized by a number of well-known histological criteria, form specialized tracts in the adult chicken atria. This reconstruction revealed a loosely arranged network of Purkinje cells connecting the two atria. This network has not been described before. No tracts could be detected between the sinoatrial and the atrioventricular nodes. These atrial Purkinje cells express the atrial and ventricular myosin isoform, as determined by the use of monoclonal antibodies that were prepared against atrial and ventricular myosin isoform, respectively. Some atrial myocytes that are topographically closely related to the Purkinje cells and that cannot be distinguished from the surrounding myocytes with conventional histological criteria, express, apart from the atrial myosin isoform, also the ventricular myosin isoform. The similar expression pattern of these two cell types and their close topographical relationship suggest the presence of a more elaborate system specialized in conduction than the well-known conductive system found with conventional histological techniques.